1. Field of the Invention
The present art relates to an optical switch in which a nonlinear optical effect produced in a nonlinear medium is used, an optical waveform measuring device, and a method for controlling an optical switch.
2. Description of the Related Art
Recently, as the capacity of optical fiber communications has increased, the bit rate of transmission systems has reached 40 Gb/s. In studies on the next generation systems, research and development of transmission of optical signals at 160 Gb/s or more per wavelength are conducted. In research and development of such transmission systems, optical waveform measuring devices (sampling oscilloscopes) that measure the waveform of signal light are mandatory to monitor and evaluate the quality of signal light.
A switch that appropriately samples signal light is used to measure the waveform of signal light with an optical waveform measuring device. In a known optical waveform measuring device, photoelectric conversion of light signals is first performed with a photoelectric converter, and converted electrical signals are electrically sampled with an electronic circuit to measure the waveform of light.
In such a known optical waveform measuring device, the performance is restricted by the speed (band) of processing electrical signals because signals are processed entirely by an electric circuit after the photoelectric converter. For example, in a case the electric circuit has a speed of processing electrical signals of 40 GHz, when signal light of 40 Gb/s or more is switched, signal light in a band exceeding 40 GHz cannot be correctly switched.
On the other hand, there is disclosed an optical signal processing technique for processing signals by controlling an optical signal with another optical signal, using a nonlinear optical effect produced in a nonlinear medium. For example, the nonlinear medium may use Japanese laid-open Patent No. 2006-184851. The response speed of a nonlinear optical effect is said to be about a femtosecond, and the speed of processing signals in an optical switch to which this effect is applied greatly exceeds the aforementioned speed of processing electrical signals. An optical waveform measuring device that can measure signal light of Tb/s can be implemented by applying this optical switch to an optical waveform measuring device.
FIG. 8 is a block diagram showing the structure of a known optical switch. A known optical switch 800 includes a polarization controller 801a, a polarization controller 801b, an optical coupler 802, a highly nonlinear fiber 803, a polarizer 804, and an optical band-pass filter 805, as shown in FIG. 8. The polarization controller 801a adjusts the polarization direction of input signal light to a polarization direction inclined 90° with respect to a pass axis of the polarizer 804.
The polarization controller 801b adjusts the polarization direction of an input sampling pulse to a polarization direction inclined about 45° with respect to the pass axis of the polarizer 804. The optical coupler 802 couples the signal light and the sampling pulse, the polarization directions of which are respectively adjusted by the polarization controller 801a and the polarization controller 801b. The highly nonlinear fiber 803 passes the signal light and the sampling pulse coupled by the optical coupler 802 and generates an intensity correlation signal of the signal light and the sampling pulse.
The polarization direction of the signal light is rotated by a nonlinear optical effect in the highly nonlinear fiber, and the signal light is output, with the polarization direction of the signal light being close to the polarization direction of the sampling pulse. The polarizer 804 has a pass axis oriented in a predetermined direction of signal light and a sampling pulse and passes only polarized components the polarization direction of which is parallel to the pass axis. The optical band-pass filter 805 passes and outputs only a polarized component of signal light out of polarized components of signal light and sampling pulse, the polarized components having passed through the polarizer 804.
In such a structure, switching of signal light is performed by controlling the timing of input of a sampling pulse into the highly nonlinear fiber 803. In a case where the optical switch 800 is applied to an optical waveform measuring device, photoelectric conversion of signal light output from the optical switch 800 is performed with a photo detector, and the waveforms of converted electrical signals are displayed to measure the waveform of signal light.